Placement of the acetabular cup in a hip replacement surgery is critical to providing a securement of the femoral portion as well as in providing an optimal range of motion for the patient. If the acetabular cup is improperly placed it can result in a limited range of motion or dislocation.
Typically the hip socket or acetabulum of a patient lies 35.degree. to 45.degree. of the midsagittal plane and is rotated forward 15.degree. to 20.degree. off the coronal plane. In hip replacement surgery usually, the cup implant is oriented in the acetabulum by using an acetabular cup positioning instrument. This instrument includes a horizontal arm that will align parallel to the long axis of a patient at a preferred abduction angle, i.e., 40.degree., and is therefore dependent upon the correct positioning of the patient. The cup placement is typically done by visually adjusting the cup to ensure that the horizontal arm of the device is approximately parallel to the long axis of the patient. This is done independent of femoral stem implant placement.
Once the abduction angle is set, the anteversion is then set by rotating the alignment device forward and then ensuring that a second arm on the cup positioner at a preset angle (typically about 20.degree.) is aligned parallel to the long axis of the patient thus selecting a predetermined or standard anteversion angle.
This method is dependent upon correct positioning of the patient and also uses "eyeballing" to position the cup at a preset angle while not taking into consideration the variations from individual to individual. This method also does not take into consideration the acetabular cup interaction with the femoral component which is important for optimizing the range of motion for an individual.
Accordingly, one object of the invention is to provide an acetabular trial that would assist in the determination of optimal positioning of the cup for the best and most normal range of motion, and for a securedly placed acetabular cup. Another object of the invention is to take into consideration the acetabular cup interaction with the femoral component is setting the acetabular cup alignment.